The present disclosure relates to data storage devices. In particular, the present disclosure relates to ferroelectric data storage devices.
Ferroelectric materials have provided additional means for storing digital data, where the binary “1” and “0” levels are represented by the electric polarization of a ferroelectric film pointing “upward” or “downward”. Storage devices based on ferroelectric storage media include Ferroelectric Random Access Memory (FeRAM) and scanning-probe storage systems (FE-Probe). In an FeRAM memory cell, the storage element includes a thin ferroelectric film sandwiched between fixed, conductive electrodes. In comparison, in an Fe-Probe device, one of the electrodes (referred to as a “tip”) is movable relative to the media. In each of these media, the ferroelectric material has a spontaneous polarization, which can be reversed by an applied electrical field. FIG. 1 is plot of polarization (“P”) versus the applied voltage (“V”), which illustrates a hysteresis loop typically attained with a ferroelectric material. As shown, when no biasing voltage is applied (i.e, V=0), the ferroelectric material has two stable points along the plot (referred to as points “a” and “b”), which exhibit opposing polarizations. Due to their high stability, the points “a” and “b” are suitable for data storage, where the points “a” and “b” may correspond to the binary “1” and “0” levels, respectively.
The hysteresis loop also identifies the coercive voltage (Vc) required to cause a change in the stored polarization charge. When writing data to a ferroelectric medium, the applied voltage, either positive or negative, must have an amplitude greater than the coercive voltage Vc of the ferroelectric material. For example, if the polarization charge of a given ferroelectric media is located at point “a”, an application of a negative voltage greater than −Vc causes the polarization of the ferroelectric material to spontaneously reverse by passing from point “a”, through point “c”, and to point “b”. If a subsequent positive voltage greater than +Vc is then applied, the polarization of the ferroelectric material will spontaneously reverse again by passing from point “b”, through point “d”, and back to point “a”. By applications of the voltages in this manner, the polarization of the ferroelectric material may reverse to stable positions, thereby allowing the binary “1” and “0” levels to be selectively written to the ferroelectric medium.
While such ferroelectric media are suitable for storing data, the data-reading techniques used with such media are based on a destructive operation, in which the read data is lost during the read operation. A read operation in a ferroelectric medium is performed by measuring the current flowing in the ferroelectric material, which is based on the polarization charge of the given material. However, current read measurements require a voltage to be applied that is greater than the coercive voltage (Vc) of the ferroelectric material. Thus, upon reading the current, the data stored in the ferroelectric material is effectively lost. As a result, current ferroelectric data storage media must be rewritten after a read operation to restore the previously written data. This typically involves storing the read data in a memory buffer, and rewriting the data back into the ferroelectric media after the read operation. This increases time and power consumption for reading and writing data onto ferroelectric data storage media.